Adjusting Shimano Hubs

I think I'm very close to having my cones adjusted. As a final test, I'm holding the valve stem at 3:00 and letting it go, hoping that, because the valve stem is often the heaviest part of the rim, the wheel will come to rest with the valve stem at the bottom. Instead, what comes to rest at the bottom of the wheel is the Mavic label on the rim which looks to be covering a seam in the rim. Is the seam routinely marked with the manufacturer's label, and is the seam usually heavier than the stem? I'm hoping so, because my hub is probably adjusted pretty well in that case.

The valve stem isn't always, or even usually, the heaviest spot on the wheel.

I just checked the 10 wheels on my 5 bikes. All are suspended with their wheels off the ground so they can rotate freely. Of these 10, 7 have the valve stem at the 12:00 position, two have the stems at 6:00 and one is at 9:00.

You will find that with many wheels there are dowels inserted into the join to help stabilise and keep the extrusion together. This means that on the rim this point is going to be the heaviest part of the assembly. Add the weight of the label as well.

It sounds like your preload on the bearings is fine, provided you have the wheels in the dropout and the quick release done up as tight as you would on the road. If you haven't, then you might find the preload is too tight when you do fit the wheels to the bike.